Cholesterol, which plays a very important role in organisms, is essential for the survival of all animal cells. Under normal physiological conditions, the cholesterol in higher biological cells is maintained at a level of a fairly narrow range. When cholesterol concentration is too high or too low, the normal life process will be affected, even serious lesions will occur. Cells maintain a normal cholesterol concentration mainly by regulating the balance between various pathways such as the synthesis, absorption, esterification, and outflow of cholesterol. Among them, the esterification of cholesterol, which is catalyzed by acyl-CoA: cholesterol acyltransferase (ACAT), plays a very important role in the balance of cholesterol metabolism both at the cell level and at the individual level. ACAT is the only enzyme in cells that synthesizes cholesteryl esters—catalyzing the formation of cholesteryl esters by connecting free cholesterol with fatty acid long chains.
ACAT is a membrane-bound protein located on the rough endoplasmic reticulum of the tissue cells. Two subtypes were found: ACAT1 and ACAT2. Both of them have different locations and distribution. ACAT1 almost exists in various tissues and cells, and regulates cholesterol levels in tissues such as brain, macrophages and adrenal glands. In contrast, ACAT2, which is expressed specifically in liver and small intestine cells, is responsible for the esterification and synthesis of cholesterol in liver and small intestine. It has long been recognized that ACAT is closely associated to the occurrence of atherosclerosis. Thus, inhibition of ACAT can not only attenuate the absorption of cholesterol by small intestine, but also inhibit the formation of multiple types of foam cells, including macrophage source, and thus it is a very important target for the treatment of cardiovascular disease.
Currently known ACAT inhibitors are mainly classified as follows: a. synthetic inhibitors: including ureas, amides, and imidazoles; b. microbial inhibitors; c. natural plant inhibitors. However, until now none of the existing ACAT inhibitors has been developed into drugs because the selectivity of inhibitory activity for the two subtypes of ACAT has been ignored. Later, different conclusions were drawn when evaluating the effect on atherosclerosis by inhibiting ACAT1. One laboratory believed that the absence of ACAT1 can inhibit the occurrence of atherosclerosis; whereas the test results from another laboratory showed that the risk of atherosclerosis is greatly increased in the ACAT1-deficient mice. It has been found in the mice without ACAT2 that ACAT-2−/− mice had lower ability to absorb cholesterol and were resistant to calculus and food-induced hypercholesterolemia. Therefore, it is predicted that specific inhibition of ACAT1 will disrupt the balance of intracellular cholesterol metabolism, leading to cytotoxicity of cholesterol, which is not helpful for preventing the occurrence of atherosclerosis. And ACAT2 may be an effective target for the prevention of hyperlipidemia and atherosclerosis. Specific inhibition of ACAT2 will reduce absorption and transport of cholesterol, and will not affect intracellular cholesterol metabolism balance. In conclusion, it is very important to develop an inhibitor with high selectivity targeting ACAT2.
However, after retesting the discovered ACAT inhibitors, it was found that only Pyripyropene A has ACAT2-specific inhibitory activity. Pyripyropenes were extracted and obtained from the fermentation broth of microorganism Aspergillus fumigates FO-1289 by Satoshi Omura et al. in 1993. It is very difficult to obtain Pyripyropenes by isolation process from natural sources, since the process involves cumbersome procedures with low production. Besides, natural Pyripyropenes have the drawback in difficult preparation. For example, starting from carvone, the synthetic route has up to nineteen steps, many of which require very harsh reaction conditions, and the yield is very low. In order to find a novel ACAT2 inhibitor with higher inhibitory activity and better selectivity, the present inventor tried to simplify the structure of Pyripyropenes by removing the ring structure in the nucleus of Pyripyropenes, which perhaps is most complicated to synthesize in the preparing process, eliminating the two methylene groups and one corner methyl group in the leftmost ring, and retaining the key diacetyl structure unit, thereby obtaining a structurally-simplified target molecule with a whole new skeleton, whose structural complexity is greatly reduced and thus is easy to produce from the simple natural ingredient carvone. A class of tricyclic compounds of Pyripyropenes disclosed by the present invention is characterized in that, comparing with the natural product Pyripyropene A, the ACAT2 specific inhibitory activity of the compounds, which are obtained by removing the leftmost ring system, increases greatly. The invention discloses the effect of this structure on the activity and obtains a series of compounds with excellent properties. When compared with the natural product Pyripyropene A, these compounds are not only simple to synthesize, but also significantly better in the aspects of inhibitory activity of ACAT2 and selective inhibition of ACAT2. They are expected to be developed into novel ACAT2-targeting drugs for the treatment of cardiovascular diseases such as atherosclerosis and the like.